NMR Study of CHN Hydrogen Bond and Proton Transfer in 1,1-Dinitroethane Complex with 2,4,6-Trimethylpyridine

The intermolecular complex with a CHN hydrogen bond formed by 1,1-dinitroethane (DNE) and 2,4,6-trimethylpyridine (collidine) dissolved in CD₂Cl₂ was studied experimentally by ¹H NMR spectroscopy at 180–300 K. Equilibrium between the molecular CH···N form and the zwitterionic C^–/HN⁺ form was detected in the slow exchange regime in the NMR time scale. No sign of a direct C^–···HN⁺ bond was observed; the ion pair is likely to be held by Coulomb interactions. Moreover, there are indications that the protonated base is involved in the formation of homoconjugated (NHN)⁺ collidine–collidinium hydrogen bonded complexes. The reaction pathway of proton transfer in the DNE–pyiridine complex in a vacuum was studied computationally at the B3LYP/6-31++G­(d,p) level of theory. NMR chemical shifts and coupling constants were calculated for a series of snapshots along the proton transfer coordinate. While the central carbon atom has a pyramidal (sp³) configuration in DNE, it is flat (sp²) in the DNE carbanion. As a result, the most indicative computed NMR parameter reflecting hybridization of a carbon atom appeared to be ¹<i>J</i>_CC, which starts to change rapidly as soon as a structure with a quasi-symmetric C··H··N bond is reached. Couplings within the hydrogen bridge, ¹<i>J</i>_CH, ^1h<i>J</i>_HN, and ²<i>J</i>_CN, can serve as good indicators of the degree of proton transfer

NMR Studies of Solvent-Assisted Proton Transfer in a Biologically Relevant Schiff Base: Toward a Distinction of Geometric and Equilibrium H-Bond Isotope Effects

The tautomeric equilibrium in a Schiff base, N-(3,5-dibromosalicylidene)-methylamine 1, a model for the hydrogen bonded structure of the cofactor pyridoxal-5‘-phosphate PLP which is located in the active site of the enzyme, was measured by means of 1H and 15N NMR and deuterium isotope effects on 15N chemical shifts at variable temperature and in different organic solvents. The position of the equilibrium was estimated using the one-bond 1J(OHN) and vicinal 3J(HαCNH) scalar coupling constants. Additionally, DFT calculations of a series of Schiff bases, N-(R1-salicylidene)-alkyl(R2)amines, were performed to obtain the hydrogen bond geometries. The latter made it possible to investigate a broad range of equilibrium positions. The increase of the polarity of the aprotic solvent shifts the proton in the intramolecular OHN hydrogen bond closer to the nitrogen. The addition of methanol and of hexafluoro-2-propanol to 1 in aprotic solvents models the PLP−water interaction in the enzymatic active site. The alcohols, which vary in acidity and change the polarity around the hydrogen bond, also stabilize the equilibrium, so that the proton is shifted to the nitrogen

NMR Studies of Coupled Low- and High-Barrier Hydrogen Bonds in Pyridoxal-5‘-phosphate Model Systems in Polar Solution

The 1H and 15N NMR spectra of several 15N-labeled pyridoxal-5‘-phosphate model systems have been measured at low temperature in various aprotic and protic solvents of different polarity, i.e., dichloromethane-d2, acetonitrile-d3, tetrahydrofuran-d8, freon mixture CDF3/CDClF2, and methanol. In particular, the 15N-labeled 5‘-triisopropyl-silyl ether of N-(pyridoxylidene)-tolylamine (1a), N-(pyridoxylidene)-methylamine (2a), and the Schiff base with 15N-2-methylaspartic acid (3a) and their complexes with proton donors such as triphenylmethanol, phenol, and carboxylic acids of increasing strength were studied. With the use of hydrogen bond correlation techniques, the 1H/15N chemical shift and scalar coupling data could be associated with the geometries of the intermolecular O1H1N1 (pyridine nitrogen) and the intramolecular O2H2N2 (Schiff base) hydrogen bonds. Whereas O1H1N1 is characterized by a series of asymmetric low-barrier hydrogen bonds, the proton in O2H2N2 faces a barrier for proton transfer of medium height. When the substituent on the Schiff base nitrogen is an aromatic ring, the shift of the proton in O1H1N1 from oxygen to nitrogen has little effect on the position of the proton in the O2H2N2 hydrogen bond. By contrast, when the substituent on the Schiff base nitrogen is a methyl group, a proton shift from O to N in O1H1N1 drives the tautomeric equilibrium in O2H2N2 from the neutral O2−H2···N2 to the zwitterionic O2-···H2−N2+ form. This coupling is lost in aqueous solution where the intramolecular O2H2N2 hydrogen bond is broken by solute−solvent interactions. However, in methanol, which mimics hydrogen bonds to the Schiff base in the enzyme active site, the coupling is preserved. Therefore, the reactivity of Schiff base intermediates in pyridoxal-5‘-phosphate enzymes can likely be tuned to the requirements of the reaction being catalyzed by differential protonation of the pyridine nitrogen

NMR Studies of Solvent-Assisted Proton Transfer in a Biologically Relevant Schiff Base: Toward a Distinction of Geometric and Equilibrium H-Bond Isotope Effects

The tautomeric equilibrium in a Schiff base, N-(3,5-dibromosalicylidene)-methylamine 1, a model for the hydrogen bonded structure of the cofactor pyridoxal-5‘-phosphate PLP which is located in the active site of the enzyme, was measured by means of 1H and 15N NMR and deuterium isotope effects on 15N chemical shifts at variable temperature and in different organic solvents. The position of the equilibrium was estimated using the one-bond 1J(OHN) and vicinal 3J(HαCNH) scalar coupling constants. Additionally, DFT calculations of a series of Schiff bases, N-(R1-salicylidene)-alkyl(R2)amines, were performed to obtain the hydrogen bond geometries. The latter made it possible to investigate a broad range of equilibrium positions. The increase of the polarity of the aprotic solvent shifts the proton in the intramolecular OHN hydrogen bond closer to the nitrogen. The addition of methanol and of hexafluoro-2-propanol to 1 in aprotic solvents models the PLP−water interaction in the enzymatic active site. The alcohols, which vary in acidity and change the polarity around the hydrogen bond, also stabilize the equilibrium, so that the proton is shifted to the nitrogen

Solvent and H/D Isotope Effects on the Proton Transfer Pathways in Heteroconjugated Hydrogen-Bonded Phenol-Carboxylic Acid Anions Observed by Combined UV–vis and NMR Spectroscopy

Heteroconjugated hydrogen-bonded anions A···H···X^– of phenols (AH) and carboxylic/inorganic acids (HX) dissolved in CD₂Cl₂ and CDF₃/CDF₂Cl have been studied by combined low-temperature UV–vis and ¹H/¹³C NMR spectroscopy (UVNMR). The systems constitute small molecular models of hydrogen-bonded cofactors in proteins such as the photoactive yellow protein (PYP). Thus, the phenols studied include the PYP cofactor 4-hydroxycinnamic acid methyl thioester, and the more acidic 4-nitrophenol and 2-chloro-4-nitrophenol which mimic electronically excited cofactor states. It is shown that the ¹³C chemical shifts of the phenolic residues of A···H···X^–, referenced to the corresponding values of A···H···A^–, constitute excellent probes for the average proton positions. These shifts correlate with those of the H-bonded protons, as well as with the H/D isotope effects on the ¹³C chemical shifts. A combined analysis of UV–vis and NMR data was employed to elucidate the proton transfer pathways in a qualitative way. Dual absorption bands of the phenolic moiety indicate a double-well situation for the shortest OHO hydrogen bonds studied. Surprisingly, when the solvent polarity is low the carboxylates are protonated whereas the proton shifts toward the phenolic oxygens when the polarity is increased. This finding indicates that because of stronger ion-dipole interactions small anions are stabilized at high solvent polarity and large anions exhibiting delocalized charges at low solvent polarities. It also explains the large acidity difference of phenols and carboxylic acids in water, and the observation that this difference is strongly reduced in the interior of proteins when both partners form mutual hydrogen bonds

NMR Study of Conformational Exchange and Double-Well Proton Potential in Intramolecular Hydrogen Bonds in Monoanions of Succinic Acid and Derivatives

We present a 1H, 2H, and 13C NMR study of the monoanions of succinic (1), meso- and rac-dimethylsuccinic (2, 3), and methylsuccinic (4) acids (with tetraalkylammonium as the counterion) dissolved in CDF3/CDF2Cl at 300–120 K. In all four monoanions, the carboxylic groups are linked by a short intramolecular OHO hydrogen bond revealed by the bridging-proton chemical shift of about 20 ppm. We show that the flexibility of the carbon skeleton allows for two gauche isomers in monoanions 1, 2, and 4, interconverting through experimental energy barriers of 10–15 kcal/mol (the process itself and the energy barrier are also reproduced in MP2/6-311++G** calculations). In 3, one of the gauche forms is absent because of the steric repulsion of the methyl groups. In all four monoanions, the bridging proton is located in a double-well potential and subject, at least to some extent, to proton tautomerism, for which we estimate the two proton positions to be separated by ca. 0.2 Å. In 1 and 3, the proton potential is symmetric. In 2, slowing the conformational interconversion introduces an asymmetry to the proton potential, an effect that might be strong enough even to synchronize the proton tautomerism with the interconversion of the two gauche forms. In 4, the asymmetry of the proton potential is due to the asymmetric substitution. The intramolecular H-bond is likely to remain intact during the interconversion of the gauche forms in 1, 3, and 4, whereas the situation in 2 is less clear

PO Moiety as an Ambidextrous Hydrogen Bond Acceptor

Hydrogen bond patterns of crystals of phosphinic, phosphonic, and phosphoric acids and their cocrystals with phosphine oxides were studied using ³¹P NMR and single-crystal X-ray diffraction. Two main factors govern these patterns and favor or prevent the formation of cocrystals. The first one is a high proton-accepting ability of the PO moiety in these acids. As a result, this moiety effectively competes with other proton acceptors for hydrogen bonding. For example, this moiety is a stronger proton acceptor than the CO moiety of carboxylic acids. The second factor is the inclination of the PO moiety of both the acids and the oxides to form two hydrogen bonds at once. The peculiarity of these bonds is that they weaken each other to a little degree only. In order to highlight this point, we are using the term “ambidextrous”. These two features should govern the interactions of PO moiety with water and other proton donors and acceptors in molecular clusters, the active sites of enzymes, soft matter, and at surfaces

Low-Temperature NMR Studies of the Structure and Dynamics of a Novel Series of Acid−Base Complexes of HF with Collidine Exhibiting Scalar Couplings Across Hydrogen Bonds^†

The low-temperature 1H, 19F, and 15N NMR spectra of mixtures of collidine-15N (2,4,6-trimethylpyridine-15N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the temperature range 94−170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19F1H19F and 19F1H15N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex [ColHFHCol]+[FHF]- is a contact ion pair. Collidinium hydrogen difluoride [ColH]+[FHF]- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the [FHF]- anion. In this complex, the anion [FHF]- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride [ColH]+[F(HF)2]- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the [F(HF)n]- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant 1JFH when the F···H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1JFH = 0, the heavy atom coupling constant 2JFN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries

Nuclear Magnetic Resonance and ab Initio Studies of Small Complexes Formed between Water and Pyridine Derivatives in Solid and Liquid Phases

The structure and geometry of hydrogen-bonded complexes formed between heterocyclic bases, namely, pyridine and 2,4,6-trimethylpyridine (collidine), and water were experimentally studied by NMR spectroscopy in frozen phase and in highly polar aprotic liquefied freon mixtures and theoretically modeled for gas phase. Hydrogen-bonded species in frozen heterocycle−water mixtures were characterized experimentally using 15N NMR. When base was in excess, one water molecule was symmetrically bonded to two heterocyclic molecules. This complex was characterized by the rHN distances of 1.82 Å for pyridine and 1.92 Å for collidine. The proton-donating ability of water in such complexes was affected by an anticooperative interaction between the two coupled hydrogen bonds and exhibited an apparent pKa value of about 6.0. When water was in excess, it formed water clusters hydrogen bonded to base. Theoretical analysis of binding energies of small model heterocycle−water clusters indicated that water in such clusters was oriented as a chain. The NMR estimated rHN distances in these species were 1.69 Å for pyridine and 1.64 Å for collidine. Here, the proton-donating ability of the hydroxyl group bonded to the heterocycle was affected by a mutual cooperative interaction with other water molecules in the chain and became comparable to the proton-donating ability of a fictitious acid, exhibiting an apparent pKa value of about 4.9. This value seems to depend only slightly on the length of the water chain and on the presence of another base at the other end of the chain if more than two water molecules are involved. Thus, the proton-donating ability of the outer hydroxyl groups of biologically relevant water bridges should be comparable to the proton-donating ability of a fictitious acid exhibiting a pKa value of about 4.9 in water. Driven by the mixing entropy, monomeric water presented in the aprotic freonic mixtures above 170 K but completely precipitated upon further cooling. Traces of water could be suspended in the mixtures down to 130 K in the presence of about 20-fold excess of heterocyclic bases. The obtained experimental data indicated that at these conditions water trended to form the symmetric 2:1 heterocycle−water complexes, whose bridge protons resonated around 6.7 ppm